The invention relates to a process for preparing liquid hydrocarbons.
It is known to prepare liquid hydrocarbons (having a boiling point above ambient temperature at a pressure of 1 bar) from an olefins-containing feed with a solid oligomerization catalyst at oligomerization conditions, to separate the normally liquid products from unconverted feed and to recycle said feed to the oligomerization zone in order to increase the yield of liquid products.
A disadvantage of the known process is that the composition of the liquid products thus produced is more or less fixed, i.e., depending on the type of catalyst and process conditions applied the product slate comprises certain fractions of gasoline, middle distillates (comprising kerosine and naphtha) and a usually small fraction of still higher boiling compounds.
Moreover, in some cases the stability of the catalyst is not as high as would be desired for a commercial operation, probably due to the deposition of some of the highest boiling compounds on the catalyst.
An object of the invention is to provide a process which is very flexible towards the preparation of various desired liquid hydrocarbon fractions, at stable operating conditions and without loss of olefins conversion, compared with known processes.
Surprisingly, it has now been found that specific desired liquid hydrocarbon fractions can be prepared by separating effluent from an oligomerization zone into at least two fractions having different boiling ranges, and recycling at least part of a fraction having a boiling range above that of the feed olefins to the oligomerization zone.